In electrically driven thermoacoustic refrigerators, and in other applications, a need exists to couple reciprocating mechanical work, produced by an electromechanical transducer to the working gas. In engines, one can conceive of the reverse process. An example of usage within thermoacoustic machines has been described in U.S. Pat. No. 6,725,670. Several examples exist of this type of structure. Conventional corrugated metal bellows have often been used. In general, these metal bellows structures begin as a sheet product, are welded into a tubular shape, and the corrugations are subsequently formed. These forming and assembly operations place some restrictions on the types of material that can be used, and production of corrugated metal bellows with high endurance limit materials can become expensive. Materials like Inconel 718 and 17-7Ph stainless steel have been used for the construction of bellows, which have high endurance limits in their final condition, and yet are ductile and weldable; not inexpensive as raw materials, these materials further rely for their high strength on lengthy (and thus costly) precipitation hardening heat treatments. This has encouraged the development of alternatives for thermoacoustic applications and U.S. Pat. No. 6,755,027 describes a recent novel alternative structure to accomplish this task. The subject of the current invention is application of a new bellows structure to perform this function.
For purposes of this disclosure, a bellows is defined as any tube-like conveyances or compliant cavities with substantial compliance along the axis of motion, which provide a boundary over which a pressure difference can be maintained and which are intended in operation to undergo axial compression, as a desired part of their function. As such, this definition includes devices such as described in U.S. Pat. Nos. 6,237,922, 6,725,670, and 6,755,027, the entire contents of all of which are incorporated herein by reference. This definition also includes other devices not specifically named as bellows, as would be recognized by a person of skill in the art. “Conventional bellows” will be used to refer to bellows with essentially uniform stiffness properties along the axis of the bellows structure, as is well approximated by a number of identical repeated structural elements along the compression axis of the bellows. When referring to “formed bellows”, aspects relevant to the sort of corrugated structures described by the Standards of the Expansion Joint Manufacturers are described. Bellows may be formed from a variety of materials, including metal and elastomeric material, and may be formed in a variety of ways.
The field of use of bellows seals on reciprocating pistons is quite old and well developed. In reciprocating machine applications, wherein part of the expected operation of the bellows is that it undergo axial compression and yet act as a piston seal at relatively high frequencies, problems associated with the relatively slow wave speed in bellows materials are a recognized issue, and various methods have been devised to compensate. (For a discussion of the problems, and methods to compensate see, for example, U.S. Pat. Nos. 3,875,806 to Brewster and 4,556,369 to Braun, wherein external devices are employed to stabilize the bellows and multiple attachments are made to the bellows, intermediate between the ends.)
The recognized problem can be summarized in this manner: bellows are designed to be compliant structures, and yet they have non-trivial mass, so that the propagation rate of a disturbance initiated at one end, traveling through the bellows is much slower than in the bulk media of which the bellows are constructed. As such, standing waves form and this causes stress distributions that are locally higher and different in distribution from what is observed in the static compression or extension case. In general, for applications in reciprocating machinery, it is desirable to ensure that the stresses in the bellows are minimized so that the part will have the greatest fatigue resistance, and thus a large lifetime.